Which physical property affects the particle size of a substance?

• A. Viscosity
• B. Melting point
• C. Molecular weight
• D. Vapor density

Answer: (C)

Molecular weight is a key factor that influences the particle size of a substance. It refers to the mass of a molecule and is determined by the number and types of atoms present in that molecule. As molecular weight increases, it typically correlates with larger molecular structures, which can lead to larger particle sizes. For example, larger molecules may aggregate together to form larger particles, while smaller molecules may remain as individual entities.

In many materials, the molecular weight can also determine how substances interact with one another. Higher molecular weight substances might have more complex structures, resulting in different physical characteristics, including size.

The other properties listed do not have a direct impact on particle size. Viscosity pertains to a fluid's resistance to flow rather than its physical structure. Melting point is the temperature at which a solid becomes a liquid and does not directly correlate to the size of the particles in a solid phase. Vapor density refers to the density of a vapor in comparison to air and does not relate to the size of particles in a substance, especially in solid or liquid forms. Therefore, understanding molecular weight is crucial in determining the size of particles present in various substances.